Working with Lists

 

    In this blog you’ll learn how to loop through an entire list using just a few lines of code regardless of how long the list is. Looping allows you to take the same action, or set of actions, with every item in a list. As a result, you’ll be able to work efficiently with lists, slice them as per need and also learn about tuples.

Looping Through an Entire List

When you want to do the same action with every item in a list, you can use Python’s for loop. Let’s say we have a list of different foods’ names, and we want to print out each name in the list. We could do this by retrieving each name from the list individually, but this approach could cause several problems. For one, it would be repetitive to do this with a long list of names. Also, we’d have to change our code each time the list’s length changed. A for loop avoids both of these issues by letting Python manage these issues internally.

foods = ['rice', 'pizza', 'pasta', 'bread'] 

for food in foods:

    print(food)

A Closer Look at Looping

The concept of looping is important because it’s one of the most common ways a computer automates repetitive tasks. When you’re using loops for the first time, keep in mind that the set of steps is repeated once for each item in the list, no matter how many items are in the list. If you have a million items in your list, Python repeats these steps a million times—and usually very quickly.

Doing Something Within and After a for Loop

foods = ['rice', 'pizza', 'pasta', 'bread'] 

for food in foods:

    print(food.title() + " is a great food.")

print("Which food do you like the most?")

Avoiding Indentation Errors

Python uses indentation to determine when one line of code is connected to the line above it. In the previous examples, the lines that printed messages to individual magicians were part of the for loop because they were indented. Python’s use of indentation makes code very easy to read. Basically, it uses whitespace to force you to write neatly formatted code with a clear visual structure. In longer Python programs, you’ll notice blocks of code indented at a few different levels. These indentation levels help you gain a general sense of the overall program’s organization.

Let’s examine some of the more common indentation errors.

Forgetting to Indent

foods = ['rice', 'pizza', 'pasta', 'bread'] 

for food in foods:

print(food.title() + " is a great food.")

Indenting Unnecessarily

message = "Hello Python world!" 

print(message)

Forgetting the Colon

foods = ['rice', 'pizza', 'pasta', 'bread'] 

for food in foods

    print(food.title() + " is a great food.")

Making Numerical Lists

Lists are ideal for storing sets of numbers, and Python provides a number of tools to help you work efficiently with lists of numbers.

Using the range() Function

Python’s range() function makes it easy to generate a series of numbers. For example, you can use the range() function to print a series of numbers like this:

for value in range(1,5): 

    print(value)

In this example, range() prints only the numbers 1 through 4. This is another result of the off-by-one behavior you’ll see often in programming languages. The range() function causes Python to start counting at the first value you give it, and it stops when it reaches the second value you provide. Because it stops at that second value, the output never contains the end value, which would have been 5 in this case. Here to print the numbers from 1 to 5, you would use range(1,6).

Using range() to Make a List of Numbers

If you want to make a list of numbers, you can convert the results of range() directly into a list using the list() function. When you wrap list() around a call to the range() function, the output will be a list of numbers.

numbers = list(range(1,6)) 

print(numbers) gives [1, 2, 3, 4, 5]

We can also use the range() function to tell Python to skip numbers in a given range. For example, here’s how we would list the even numbers between 1 and 10:

even_numbers = list(range(2,11,2)) 

print(even_numbers) gives [2, 4, 6, 8, 10]

For example, let's make a list of squares of above even numbers.

even_squares = []

for even_number in list(range(2,11,2)): 

    even_squares.append(even_number**2)

print(even_squares) gives [4,16,36,64,100]

Simple Statistics with a List of Numbers

A few Python functions are specific to lists of numbers. For example, you can easily find the minimum, maximum, and sum of a list of numbers:

digits = [1, 2, 3, 4, 5, 6, 7, 8, 9, 0]

print(min(digits)) gives 0

print(max(digits)) gives 9

print(sum(digits)) gives 45

List Comprehensions

The approach described earlier for generating the list squares consisted of using three or four lines of code. A list comprehension allows you to generate this same list in just one line of code. A list comprehension combines the for loop and the creation of new elements into one line, and automatically appends each new element.

The following example builds the same list of square numbers you saw earlier but uses a list comprehension:

even_squares = [value**2 for value in range(2,11,2)] 

print(even_squares) gives [4,16,36,64,100]

To use this syntax, begin with a descriptive name for the list, such as even_squares. Next, open a set of square brackets and define the expression for the values you want to store in the new list. In this example the expression is value**2, which raises the value to the second power. Then, write a for loop to generate the numbers you want to feed into the expression, and close the square brackets. Notice that no colon is used at the end of the for statement.

Working with Part of a List

Slicing a List

To make a slice, you specify the index of the first and last elements you want to work with. As with the range() function, Python stops one item before the second index you specify.

players = ['charles', 'martina', 'michael', 'florence', 'eli'] 

print(players[1:4]) gives ['martina', 'michael', 'florence']

If you omit the first index in a slice, Python automatically starts your slice at the beginning of the list:

print(players[:4]) gives ['charles', 'martina', 'michael', 'florence']

If you omit the last index in a slice, Python automatically slices upto the ending of the list:

print(players[2:]) gives ['michael', 'florence', 'eli'] 

This syntax allows you to output all of the elements from any point in your list to the end regardless of the length of the list. Recall that a negative index returns an element a certain distance from the end of a list; therefore, you can output any slice from the end of a list. 

For example, if we want to output the last three players on the roster, we can use the slice players[-3:]:

print(players[-3:]) gives ['michael', 'florence', 'eli'] 

This prints the names of the last three players and would continue to work as the list of players changes in size.

Looping Through a Slice

You can use a slice in a for loop if you want to loop through a subset of the elements in a list. In the next example we loop through the first three players and print their names as part of a simple roster:

players = ['charles', 'martina', 'michael', 'florence', 'eli'] 

print("Here are the first three players on my team:") 

for player in players[:3]: 

     print(player.title()) 

Copying a List

Often, you’ll want to start with an existing list and make an entirely new list based on the first one. Many programmer face some issues while copying a list, we will learn to avoid it. First let's do it the intuitive but wrong way.

my_foods = ['pizza', 'carrot cake']

friend_foods = my_foods

my_foods.append('burger')

friend_foods.append('pasta')

print(my_foods) gives ['pizza',  'carrot cake', 'burger', 'pasta']

print(friend_foods) gives ['pizza', 'carrot cake', 'burger', 'pasta']

Here both burger and pasta is added to both lists but we wanted to treat it individually.

The right way is:

my_foods = ['pizza', 'carrot cake']

friend_foods = my_foods[:]

my_foods.append('burger')

friend_foods.append('pasta')

print(my_foods) gives ['pizza',  'carrot cake', 'burger']

print(friend_foods) gives ['pizza', 'carrot cake', 'pasta']

Tuples

Lists work well for storing sets of items that can change throughout the life of a program. The ability to modify lists is particularly important when you’re working with a list of users on a website or a list of characters in a game. However, sometimes you’ll want to create a list of items that cannot change. Tuples allow you to do just that. Python refers to values that cannot change as immutable, and an immutable list is called a tuple.

Defining a Tuple

A tuple looks just like a list except you use parentheses instead of square brackets. Once you define a tuple, you can access individual elements by using each item’s index, just as you would for a list. 

dimensions = (200, 50)

print(dimensions[0]) gives 200

for dimension in dimensions: 

    print(dimension) 

You will get an error if you try to run code below as the elements of tuple cannot be changed.

dimensions[0] = 300

Although you can’t modify a tuple, you can assign a new value to a variable that holds a tuple. So if we wanted to change our dimensions, we could redefine the entire tuple:

dimensions = (200, 50)

print(dimensions) gives (200, 50)

dimensions = (400, 100) 

print(dimensions) gives (400, 100) 

Conclusion

In this blog you learned how to work efficiently with the elements in a list. You learned how to work through a list using a for loop, how Python uses indentation to structure a program, and how to avoid some common indentation errors. You learned to make simple numerical lists, as well as a few operations you can perform on numerical lists. You learned how to slice a list to work with a subset of items and how to copy lists properly using a slice. You also learned about tuples, which provide a degree of protection to a set of values that shouldn’t change.

In next blog, you’ll learn to respond appropriately to different conditions by using if statements. You’ll learn to string together relatively complex sets of conditional tests to respond appropriately to exactly the kind of situation or information you’re looking for. You’ll also learn to use if statements while looping through a list to take specific actions with selected elements from a list.